1. Field of the Invention
The present invention relates to an apparatus and method for removing mercury contained in a gas discharged from a boiler or a similar apparatus (hereinafter may be referred to as a discharge gas treatment apparatus and a discharge gas treatment method).
2. Background Art
Known methods for removing mercury contained in a discharge gas produced from a boiler, a gas turbine, a combustion furnace, etc. include a treatment method employing an adsorbent such as activated carbon and a gas-liquid contact method in which a liquid absorbent is sprayed onto a discharge gas in a scrubber.
Japanese Patent Application Laid-Open (kokai) No. 10-230137 discloses a discharge gas treatment method for removing mercury and NOx. In the method, a mercury-chlorinating agent and ammonia are added to a discharge gas, whereby mercury chloride is formed from metallic mercury and NOx is removed, in the presence of a solid catalyst. Subsequently, wet-format NOx, removal is performed by use of an alkaline liquid absorbent.
In the method, mercury chloride is formed through the following scheme (1):2Hg+4HCl+O2→2HgCl2+2H2  (1).The thus-formed mercury chloride is dissolved in water, to thereby remove mercury contained in the discharge gas.
Japanese Patent Application Laid-Open (kokai) No. 2000-197811 discloses a discharge gas mercury removal method. In the method, a discharge gas containing water-insoluble element-form mercury is brought into contact with a layer which is filled with at least one of a metal chloride, a metal bromide, a metal fluoride, and a metal iodide in the form of pellets or a honeycomb structure, whereby a water-soluble mercury species is formed. The thus-formed water-soluble mercury species is dissolved in a solution, to thereby remove mercury contained in the discharge gas.
Japanese Patent Application Laid-Open (kokai) No. 2003-53142 discloses an apparatus for removing mercury contained in a discharge gas. The apparatus employs a catalytic oxidation apparatus in which hydrochloric acid contained in the discharge gas is reacted with mercury in the presence of a catalyst at low temperature (300° C. or lower), to thereby form mercury chloride. The thus-formed mercury chloride is dissolved in water, whereby mercury contained in the discharge gas is removed.
However, when the aforementioned method employing an adsorbent such as activated carbon is employed, merely a small amount of mercury is adsorbed on the adsorbent. Therefore, when a large-volume discharge gas is treated, an excessive amount of mercury cannot be treated by the adsorbent.
When the above gas-liquid (discharge gas-liquid absorbent) contact method is employed, mercury chloride having high solubility in the liquid absorbent can be removed. In contrast, a large amount of metallic mercury, having low solubility in the liquid absorbent, cannot be removed, and metallic mercury must be removed by means of another apparatus.
In the discharge gas treatment method disclosed in Japanese Patent Application Laid-Open (kokai) No. 10-230137, addition of a mercury-chlorinating agent elevates treatment cost. In addition, unreacted chlorinating agent may corrode or break an apparatus such as a heat exchanger or an absorption-desulfurization tower, which is disposed on the downstream side of the gas flow.
The method of removing mercury contained in a discharge gas disclosed in Japanese Patent Application Laid-Open (kokai) No. 2000-197811 must employ a layer filled with a metal chloride or a similar material, leading to an increase in treatment cost. When the metal chloride powder is employed in the layer, a collection apparatus such as a collection filter for collecting reacted powder must be disposed, whereas when the metal chloride solution is used in the layer, a collection tank for collecting a solution after reaction must be disposed. As a result, apparatus cost increases.
The method of removing mercury contained in a discharge gas disclosed in Japanese Patent Application Laid-Open (kokai) No. 2003-53142 employs an additional catalytic oxidation apparatus. Therefore, apparatus cost is elevated.